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U.S. Route 101 (US 101) in the state of California is one of the last remaining and longest U.S. Routes still active in the state, and the longest highway of any kind in California.[4] US 101 was also one of the original national routes established in 1926. Significant portions of US 101 between the Los Angeles area and the San Francisco Bay Area follow El Camino Real, the historic road connecting the former Alta California's 21 missions.

Los Angeles to Ventura

US 101 northbound as it enters downtown Los Angeles

The south terminus of US 101 is in Los Angeles, about one mile (1.6 km) east of downtown Los Angeles at the East Los Angeles Interchange, also known as the "Commuters' Complex". This southernmost portion is named the Santa Ana Freeway, inheriting that title as the northerly extension of the roadway now known as I-5.

US 101 then intersects with SR 134 and SR 170 at the interchange known as the Hollywood Split. Here, the alignment of US 101 shifts to the alignment of SR 134 (i.e. heading northbound, the road's alignment turns left, or westbound) and thereafter is referred to as the Ventura Freeway until it reaches Ventura. Though confusing, the "Hollywood Freeway" name continues northward from this interchange on SR 170, and the "Ventura Freeway" name continues eastward to SR 134.

From the Hollywood Split, US 101 is an east-west highway (until it reaches Gaviota State Park in Santa Barbara County where it shifts back to a north-south alignment). It meets with I-405 in Sherman Oaks, an interchange which holds claim to the most traveled intersection in the nation.[] The east-west geographical alignment of the Ventura Freeway and the north-south designation which appears on the freeway signs can be confusing to visitors; the same freeway entrance can often be signed as "101 North" and "101 West"; this is most common in the San Fernando Valley where the local E/W signing does not match the Caltrans' proper statewide N/S designation.

Central Coast

North of Santa Barbara, US 101 switches intermittently between freeway and expressway status (i.e. there is occasional cross-traffic), but there are no traffic signals until San Francisco. The last traffic signals along this stretch of the route were removed in the early 1990s when the section through downtown Santa Barbara was constructed to freeway standards after years of disagreement over the impact that the original elevated design would have on the community.[9]

From Ventura and through Santa Barbara, US 101 closely follows the Pacific coastline (generally no more than one to two miles [1.6 to 3.2 km] from the shore) until Gaviota State Park, about 23 miles (37 km) west of Goleta. At Gaviota State Park, the highway shifts back from an east-west highway to a north-south alignment. About one mile (1.6 km) north of this point, US 101 passes through the Gaviota Tunnel.

A few miles north of the Gaviota Tunnel, SR 1 splits from US 101 and heads northwest, running along the Pacific coastline parallel and to the west of US 101. US 101 passes through Buellton, Los Alamos, Orcutt, Santa Maria, and Nipomo. South of Santa Maria, US 101 widens from a four-lane highway to a six-lane freeway. SR 166 joins US 101 for about 3 miles (4.8 km) before splitting just north of the city limits, while US 101 continues as a four-lane freeway before reverting to expressway status north of Nipomo.

Farther north, SR 1 rejoins US 101 between Pismo Beach and San Luis Obispo. Then US 101 takes an inland route through the Salinas Valley, while Highway 1 heads northwest, running along the Pacific coastline in California, parallel and to the west of US 101.

From Paso Robles to Salinas, US 101 is an expressway known as the Salinas River Valley Highway, since the Salinas River Valley extends from Santa Margarita to the SR 156 junction in Prunedale. US 101 resumes freeway status between San Miguel and King City, passing through the smaller towns of Camp Roberts, Bradley, and San Ardo, as well as the San Ardo Oil Field about five miles (8.0 km) south of San Ardo. Near this point, the wide agricultural bottomlands of the Salinas Valley begins. North of King City, US 101 once again switches intermittently between freeway and expressway status, passing through Greenfield, Soledad, Gonzales, and Chualar before reaching Salinas. Shortly after leaving Salinas, US 101 joins SR 156 in Prunedale for about eight miles (13 km). After crossing the San Benito County line, SR 156 splits from US 101 near San Juan Bautista while US 101 continues northward mostly as a four-lane highway until it reaches Gilroy.

US 101 sign in San Francisco

San Francisco Bay Area

US 101 in San Francisco between Sacramento and Clay Streets; a Muni ETI Skoda bus can be seen in the background.

From San Francisco north the highway is heavily traveled by commuters through to Windsor, just north of Santa Rosa. North of the Golden Gate Bridge, US 101 enters Marin County and is known as the Redwood Highway. After crossing the bridge, US 101 climbs the Waldo Grade and passes through the Robin Williams Tunnel, the only one of its kind in 300 miles (480 km) (the other being the Gaviota Tunnel in Santa Barbara County). Upon exiting the tunnel, it passes above the hillside town of Sausalito and descends to Richardson Bay, where SR 1 splits from the freeway and heads to the coast. US 101 then passes through Mill Valley, Corte Madera, Larkspur, San Rafael and Novato, before entering Sonoma County. The section between Novato in Marin County and Petaluma in Sonoma County changes from its original six or eight lanes through Marin to four lanes, creating a bottleneck, and is thus called the "Novato Narrows" by locals.[10] The Narrows continue up to North of Petaluma. The entire area between Novato and Petaluma is also the voter approved Novato-Petaluma Community Separator, which forbids most development.[11]

North Coast

US 101 crosses into the Mendocino County line as a freeway for one mile (1.6 km), but then narrows to an expressway through the Russian River canyon and eventually a two-lane road south of Hopland, the first time since leaving San Francisco. Just before reaching Ukiah, US 101 becomes a four-lane freeway. In the community of Calpella, SR 20 merges with US 101 for the next 15.5 miles (24.9 km) to Willits. The freeway portion ends as the combined US 101 and SR 20 ascend the 1,953-foot (595 m) Ridgewood Summit, the highest elevation along the route's 807-mile-long (1,299 km) trek through California. As 101 resumes freeway status just south of Willits, SR 20 splits from 101 shortly before 101 suddenly reverts to a 2-lane undivided freeway and eventually a highway north of Willits.

US 101 then widens to a four-lane expressway until the intersection with the western portion of SR 162, where it reverts to a two-lane road. North of Laytonville, US 101 ascends Rattlesnake summit (1,796 feet or 547 metres) before a descent to follow the South Fork Eel River and Eel River all the way to Fortuna near the mouth of the river. About 12 miles (19 km) northwest of Laytonville, US 101 becomes an undivided freeway near the community of Cummings. SR 271 is the old portion of US 101 through this area. At Leggett, US 101 meets SR 1 for the last time, and from this point until Piercy, US 101 runs along a section of highway with frequent landslides. Cal Trans bypassed the most difficult section in 2009 with unique construction of two bridges known collectively as the Confusion Hill Bridges. This project, funded by an emergency act from the State Legislature, moved the highway across the Eel River away from the troubled spots to prevent disruption in commerce and travel from infrequent, but costly, winter closures on the main transportation route to the far North Coast. North of Piercy, the freeway portion again ends and the road narrows down to two lanes, before another stretch of divided highway.

Arriving in Humboldt County, another narrow two lane portion of US 101 bisects Richardson Grove State Park. Just after the park boundary, the highway switches to a short undivided freeway and then eventually a divided freeway just before reaching Garberville. North of Garberville, US 101 reverts to an undivided freeway, which continuing north by northwest, passes through the 53,000-acre (210 km2) Humboldt Redwoods State Park, California's third largest State Park and the site of the largest remaining Redwood old growth forest in the world. A preserved portion of the original, bypassed highway route, known as the Avenue of the Giants for the huge, centuries-old redwood trees, parallels the highway for over 30 miles (48 km) in southern Humboldt County. US 101 again switches to another stretch of divided freeway near the town Pepperwood before reverting to a short, 2-mile (3 km) expressway just north of Stafford. A short freeway then runs through Rio Dell before another short, 3-mile (5 km) expressway.

Shortly before reaching the western terminus of SR 36, US 101 becomes a freeway again between Fortuna and Eureka. North of Humboldt Hill, the road enters the City of Eureka (a potential new freeway cutting through or bypass of the city was successfully blocked repeatedly). As the route traverses Eureka, the southern portion is known as "Broadway" and then as it bears east along Humboldt Bay, the Highway is aligned on a one-way couplet (4th and 5th streets). Five miles (8.0 km) later the highway leaves Eureka's northern city limit and continues north. The expressway style section between Eureka and Arcata, which is also a safety corridor, is named the "Michael J. Burns Freeway," in honor of the State Senator who was a proponent of California's Highways. The Highway becomes an unobstructed Freeway south of the center of Arcata. Proceeding north it passes the junction for SR 299 (also the western terminus for that route), in the Valley West (northernmost) part of the college town.

Parking adjacent to US 101 along the sandy bar between Freshwater Lagoon and the Pacific Ocean

North of the town of Klamath just inside Del Norte County, the highway closely follows the Pacific coast again. In Crescent City, US 101 once again separates into a one-way couplet (L and M Streets) for nine blocks. As it leaves Crescent City, US 101 becomes a divided freeway for the last time in California (built slightly to the west of the original two-lane alignment, now called Parkway Drive). As the 3-mile divided freeway portion ends, US 101 intersects the southern terminus of US 199, which heads northeast as the Redwood Highway, terminating in Grants Pass, Oregon. US 101 (no longer called the "Redwood Highway" at this point) is reduced to two lanes and continues north along the California coast until it reaches the Oregon border.

Early narrow concrete pavement survives on some former alignments like this stage route near Mission San Miguel Arcángel was designated US 101 until bypassed in 1938.

Instead of terminating in Los Angeles, US 101 once continued all the way south through San Diego to the United States-Mexico border in San Ysidro. However, this part was decommissioned on July 1, 1964, in favor of I-5. The only remnant of the old route on US 101's current alignment is a mileage sign at the Santa Barbara-Ventura county line, which lists the distance to San Diego, even though US 101 ends in Los Angeles.

Oceanside-Carlsbad freeway bypass

By the early 1950s, traffic had become very heavy on US 101 through Oceanside and Carlsbad. The US 101 freeway bypass (Oceanside-Carlsbad freeway bypass) was built in 1953 and completed in 1955 by the California Department of Public Works (now Caltrans) and brought up by the San Diego Highway Development Association on a US 80/US 101 discussion on how to resolve the huge traffic loads on US 101 in Oceanside. Today it is part of I-5 and Palomar Airport Road. It follows I-5 from Coast Highway (former Hill Street exit) in Oceanside to Palomar Airport Road in Carlsbad. From there the US 101 bypass went onto Palomar Airport Road to merge with US 101 Bus. on Carlsbad Boulevard. As it approached the Coast Highway/SR 76 exit on southbound I-5 in Oceanside, US 101 Bus. split off. US 101 Bus. followed the original US 101 through downtown Oceanside and Carlsbad as former Hill Street/Carlsbad Boulevard (CR S-21) while the US 101 freeway followed modern I-5 and Palomar Airport Road. The south end of the freeway bypass is Carlsbad Boulevard and Palomar Airport Road and the north end is I-5 and the Coast Highway/SR 76 exit. The south end was modified after US 101 was decommissioned between the East Los Angeles Interchange in Los Angeles and the Mexican border in San Ysidro. This freeway construction by the California Department of Public Works put US 101 on an all new highway route alignment to relieve Oceanside and Carlsbad of their very heavy bumper-to-bumper burdensome traffic problem. Also before the bypass in the 1950s, US 101 followed North Coast Highway (formerly Hill Street) from San Luis Rey Mission Expressway (SR 76 and north end of the Oceanside-Carlsbad freeway bypass) to Harbor Drive. From there it followed Harbor Drive to Vandergrift Boulevard, San Rafael Drive, and the freeway onramp for I-5 north near the Camp Pendleton north entrance guardhouse gate. It merges with the I-5 northbound onramp to shoot onto the southbound lanes of I-5 to follow the freeway lanes all the way to Las Pulgas Road in Camp Pendleton.

Historic Route In Orange County

An old orphaned alignment of US 101, that ran through the cities of Mission Viejo, Laguna Niguel, San Juan Capistrano, Dana Point, and San Clemente, was located in south Orange County. The old roadway, from a dead end just west of I-5 and east of the railroad tracks in Mission Viejo to Cristianitos Road in San Clemente, followed Camino Capistrano, Doheny Park Road, Coast Highway and El Camino Real. It had interchanges with I-5 and California State Route 1. It was replaced by the San Diego freeway (which US 101 became part of it in 1958) and finally replaced by I-5 in 1968.

El Camino Real: Los Angeles to San Francisco

Distinctive route markers with symbolic mission bell and shepherd's crook are seen between Los Angeles and San Jose

Significant portions of US 101 from its southern terminus to the San Francisco Bay Area is designated as the Royal Road or El Camino Real. The route roughly follows the historic trail that connected the former Alta California's 21 missions.

From the 1940s to 1991, various segments of US 101 between Los Angeles and San Francisco were upgraded to either a freeway or expressway. In the Los Angeles area, the first segment of the Hollywood Freeway through the Cahuenga Pass opened in 1940, while the segment from the San Fernando Valley to Downtown Los Angeles opened in 1954.[13] The Ventura Freeway then opened in 1960.[14] The segment of the original two-lane alignment between Emma Wood State Beach north to the Mobil Pier Undercrossing near Sea Cliff, which followed the historic Rincon Sea Level Road, was the re-signed as part of SR 1.

In the San Francisco Bay Area, US 101 was originally divided. US 101W followed the same general right-of-way of today's US 101 through the region, primarily along what was originally signed as Bayshore Boulevard. US 101E then generally followed the right-of-way taken by today's I-880 from San Jose to Oakland, then across the Carquinez Bridge to follow what is now SR 37, joining US 101W. The US 101E designation was removed by the 1940s and became SR 17 (later designated as I-880 and the westernmost section I-580), running from San Jose to Oakland and then across the Richmond-San Rafael Bridge. Meanwhile, Bayshore Boulevard was later redesignated as the US 101A bypass and then eventually upgraded to what is now the Bayshore Freeway. The first stretch that was completed between Redwood City and South San Francisco was the Bay Area's first freeway when it opened in 1947. After the entire Bayshore Freeway was completed in the early 1960s, the old alignment along the peninsula was renumbered and renamed as SR 82/El Camino Real.

Various other freeway or expressway bypasses along the California Central Coast were also built. In 1991, the last traffic signal along US 101 between Los Angeles and San Francisco was taken down in Santa Barbara.[15] The primary control city that is listed on freeway signs along northbound US 101 through the Central Coast region remains San Francisco. Although San Jose surpassed San Francisco population decades after the highway was built, there has been no push to change all the signs.

As the result of freeway revolts in San Francisco in the 1950s, a direct freeway connection through the city to the Golden Gate Bridge has never been built. The Central Freeway was completed to extend from the Bayshore Freeway to Turk Street in 1959,[16][17] before the San Francisco Board of Supervisors voted to remove the remainder of the Central Freeway and most other proposed freeways from the city's highway plan.[18] For decades, southbound traffic on US 101 flowed on the one-way Turk Street from Van Ness Avenue to the Central Freeway, while northbound traffic used the parallel Golden Gate Avenue. After the 1989 Loma Prieta earthquake damaged the structure, the segment of the Central Freeway north of Market Street was replaced with the surface-level Octavia Boulevard; traffic on US 101 was then eventually re-routed to exit south of that at Mission Street/South Van Ness Avenue. With no direct freeway along US 101 through the City of San Francisco, the old US 101E/I-880/I-580 route remains as a faster bypass through the Bay Area.

In 2003, the segment of US 101 between Morgan Hill and San Jose, also known as the Sig Sanchez Freeway, expanded to eight lanes between Cochrane Road and SR 85 exits.[19] Originally, the ten-mile segment was only four lanes.[20] The improved segment was to alleviate the consistent congestion that had expanded as far south as Masten Avenue coming from Gilroy, and as far north as Bernal Road coming from San Jose.[21][22] Traffic now typically only runs slow between the Bailey Avenue and Dunne Avenue exits.

The interchange at the beginning of I-280 and I-680 was constructed years before its completion. The two bridges, with no on ramps or off ramps stood over US 101 as a 110-foot-tall (34 m) monument to inefficiency for years in the 1970s. It became the butt of many local jokes. The highlight prank occurred in January 1976, when a 1960 Chevrolet Impala was placed on the highest bridge overnight, where it obviously would be impossible to drive. The following day, San Jose City Councilman Joe Colla was photographed standing next to the car, a photo which was circulated across many newspapers.[23] It has been suggested this stunt nudged the wheels of progress to find the funds to complete the freeway. In 2010, the interchange was named the Joe Colla Interchange.[24]

Construction on a freeway segment bypassing Willits began in 2013.[27] The bypass around Willits remained controversial because the intended route goes through protected wetlands. Construction was halted by US Army Corps of Engineers in June 2014 and work restarted the following month after Caltrans committed to extensive mitigation of the project.[28][29] The 5.9 mi (9.5 km) bypass around Willits opened to traffic in November 2016.[30] The bypass included a 1.1 mi (1.8 km) viaduct going over a flood plain.[31] The freeway segment cost $459 million to complete; 50% more than what Caltrans first reported when it opened.[32] The city of Eureka has long resisted a freeway through it.[]

Major intersections

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage. R reflects a realignment in the route since then, M indicates a second realignment, L refers an overlap due to a correction or change, and T indicates postmiles classified as temporary (for a full list of prefixes, see the list of postmile definitions).[3] Segments that remain unconstructed or have been relinquished to local control may be omitted. The numbers reset at county lines; the start and end postmiles in each county are given in the county column.